Double-patterning is a technology developed for lithography to enhance the feature density. Typically, for forming features of integrated circuits on wafers, lithography technology is used, which involves applying a photo resist, and defining patterns on the photo resist. The patterns in the patterned photo resist are first defined in a lithography mask and are implemented either by the transparent portions or by the opaque portions in the lithography mask. The patterns in the patterned photo resist are then transferred to the manufactured features.
With the increasing down-scaling of integrated circuits, the optical proximity effect posts an increasingly greater problem. When two separate features are too close to each other, the optical proximity effect may cause the features to short to each other. To solve such a problem, double-patterning technology is introduced. The closely located features are separated into two masks of a same double-patterning mask set, with both masks used to expose the same photo resist. In each of the masks, the distances between features are increased over the distances between features in the otherwise single mask, and hence the optical proximity effect is reduced, or substantially eliminated.
However, double-patterning technology cannot solve native conflict problems occurring when two cells are abutted. For example, referring to FIG. 1, a first cell includes features VDD, VSS, 2, and 4 that are closely located with distances S1′, S2′, and S3′ being smaller than the lithography limit. Therefore, the double-patterning technology may be used to increase the distances between features VDD, VSS, 2, and 4. Referring to FIG. 2, a second cell includes features VDD, VSS, and 6 that are closely located with both distances S4′ and S5′ being smaller than the lithography limit. Therefore, the double-patterning technology may also be used. However, if the first cell and the second cell are to be abutted, a problem arises. In the first cell, features VDD and 4 have to be in a same mask, and features VSS and 2 have to be in a same mask different from the mask of features VDD and 4. In the second cell, VDD and VSS have to be in a same mask that is different from the mask of feature 6. Accordingly, the first cell and the second cell have a conflicting requirement regarding whether features VDD and VSS should be in a same mask or different masks.